Some great shows survived a major star’s departure, others didn’t. The keys to success: bring in strong new characters – and avoid cute kids.

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John Krasinski and Steve Carell star in NBC's "The Office," which landed a nomination for best comedy or musical television series.

Updated at 11:57 AM CDT on Wednesday, Jun 30, 2010

Chalk it up to a lack of imagination, but we're having a hard time envisioning "The Office" without Steve Carell.

It's akin to the greatest office comedy of them all, "The Mary Tyler Moore Show," continuing without the lady mentioned in the title.

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But with Carell apparently set on leaving and NBC equally determined to keep the show going beyond his departure next year after seven seasons, it might be wise to look at how other major TV comedies survived big cast changes.

The history is mixed, but a couple keys to success emerge: No. 1, replace strong characters with new – and distinctly different – strong characters, rather than relying solely on the existing ensemble. And, No. 2, avoid, at all costs, introducing a cute kid into the fray.

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After three seasons, M*A*S*H lost two main characters, Col. Henry Blake (McLean Stevenson) and Trapper John McIntyre (Wayne Rogers) at once. Some sounded the death knell for the show, which, by then, had lasted about as long as the Korean War.

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The interplay between the characters – and the show itself – changed markedly, shifting from black humor to a deeper, humor-infused morality play that occasionally bordered on preachy. But M*A*S*H left after 11 seasons on top.

The same can be said for "Cheers," another popular-to-the-end, 11-season wonder that lost one main character to death (the late Nicholas Colasanto, who played the gentle, befuddled Coach) and another to the movies (Shelley Long, whose neurotic, pseudo-intellectual barmaid Diane Chambers drove more than her share of the action).

She may more known these days for her weight travails, but Kirstie Alley's arrival in Season 6 as ambitious but lovelorn bar manager Rebecca Howe reinvigorated “Cheers,” whose Sam-and-Diane tempestuous relationship was in danger of getting played out. Woody Harrelson, meanwhile, cut his comic teeth as Woody, Coach's lovable, dimwitted country-boy replacement behind the bar.

Both "M*A*S*H" and "Cheers," of course, were fueled by consistently great writing.

The biggest cautionary tale in forging ahead when key characters leave comes from perhaps the best-written TV comedy ever: "All in the Family." The first cute-kid insertion – the Bunker’s grandson Joey – worked in limited doses. But with the departure of Mike and Gloria Stivic (Rob Reiner and Sally Struthers) after Season 8, the show began its decline.

The contrived arrival of an abandoned young niece taken in by the Bunkers – and the subsequent, unforgivable killing of Edith while trying to keep the show alive as "Archie Bunker's Place" – stand in striking contrast to the brilliant early seasons.

If the folks behind “The Office” are at all tempted to rebuild the show in any significant way around Jim and Pam's lives as parents, they need to look no further than to Ghosts of NBC Must-See TV Thursday nights past for guidance.

The likable "Family Ties" began sputtering when the family's baby, Andy, seemed to age at several times the normal rate and began sharing syrupy scenes with Michael J. Fox' Alex Keaton character. When Keisha Knight Pulliam aged out of the cute-kid role as Rudy on "The Cosby Show," and Raven Symone was brought in as the family’s new youngster, Olivia, in Season 6, it was a signal the Huxtables were running out of laughing gas.

We're fairly confident "The Office” creative team, given their strong track record, will avoid obvious pitfalls, though generating instant chemistry between old characters and new ones is a tricky business.

The show has been somewhat scattershot since the Jim-and-Pam wedding, but the recent introduction of Kathy Bates as Dunder-Mifflin’s new owner provided some focus. Her tough, Southern matriarch proved a reminder that the show is at its best when making uncomfortable comedy out of very real fears about working life in this economy.

The very good season finale, which raised the possibility Carell’s Michael Scott could be reunited with Holly, his soulmate in silliness, bodes for a strong arc in what likely will be the actor’s final season. We’ll see whether Michael Scott mucks up his last chance for happiness – and whether “The Office” can go on without him.